HCS was digested by human plasmin as previously described for HGH. The digest was purified by gel-filtration first on Sephadex G-75 in 20% HOAc and the recovered material was next submitted to chromatography on Sephadex G-100 in a neutral buffer. The purified plasmin-modified HCS was submitted to reduction and alkylation and the product was resolved into two components by gel filtration on Sephadex G-50 in 10% acetic acid. The isolation components (designated N and C) were chromatographed on Sephadex G-100. From 100 mg HCS, a yield of highly purified N and C were 31 mg and 8 mg, repsectively. Amino acid composition of N and C indicate that N is the NH2-terminal 133 amino-acids fragment and C of the COOH terminal 141-191 fragment of HCS molecule. Amino and carboxyl terminal residue analyses were consistent with this conclusion. In addition, NH-terminal sequence of N was found to be: H-Val-Glx-Thr-Val-. Amino acid analyses of carboxypeptidase A/B digests of N and C suggested the following COOH-terminal sequences: N, -Gly-ser-Arg; C, -Gly-Phe. It may be concluded that N represents Cys(Cam)53 -HCS-(1-133) and C, Cys(Cam) 165, 182, 189, -HCS-141-191. The results of bioassay in the pigeon crop-sac test indicated that both fragments have markedly decreased in their biological activity. The large fragment may have low activity but is much less active than the native hormone. Noncovalent interaction of the plasmin fragments of reduced-carbamoylmethylated (Cam) human somatotropin (hGH) with those of reduced-carbamoylmethylated human chorionic somatommamotropin (hCS) have been investigated. It was found that the recombinant obtained by noncovalent interaction of (Cys(Cam)53)hGH-(1-134) with (Cys(Cam)165,182,189) hCS-(141-191) exhibits 50% growth-promoting activity and nearly full immunoreactivity.